The present invention relates to a method of controlling an electric motor of an electric power steering system having at least three windings and to a power steering system having an electric motor for steering assistance.
In an electric power steering system of a motor vehicle, typically an electric motor, in most cases a three-phase electric motor having three windings, is used for providing assistance to the driver during steering maneuvers. To this end, the electric motor is connected to the steering system such that it can transmit a torque to the steering linkage directly or indirectly.
The quantity of this torque, which assists the driver in steering maneuvers, is ascertained by a control unit on the basis of measured variables such as the vehicle speed, the torque in the steering shaft and/or the speed of rotation of the steering shaft. These measured variables are picked up by sensors in the vehicle.
The total torque ascertained by the control unit is normally made up of two components, more specifically the set-point torque for steering assistance and the set-point torque for damping. The set-point torque for steering assistance here constitutes the torque which assists the driver in his/her intended steering maneuver. It is mainly ascertained from the vehicle speed and the current torque in the steering shaft. Accordingly, the set-point torque for steering assistance follows the torque applied by the driver, assisting him/her in the steering process.
The set-point torque for damping improves the driving safety of the vehicle. For this purpose, in addition to the vehicle speed and the torque in the steering shaft, the speed of rotation of the steering shaft is also acquired, and a set-point torque is calculated by the control which avoids heavy steering deflections and counteracts any excessive steering movements of the driver. To this end, the set-point torque for damping may possibly counteract the set-point torque for steering assistance and the torque applied by the driver. The set-point torque for damping thus constitutes a corrective which increases the driving safety.
In the event of a malfunction of the electric motor, the steering assistance is not provided faultlessly, so that an increased steering moment has to be applied, more particularly by the driver. A possible malfunction of the electric motor here includes more particularly defects of the windings, for example a short circuit of one or more windings or a line defect of a winding, resulting in an open phase. To prevent this, the function of the electric motor, particularly that of the windings, is usually monitored, and in the event of a motor defect, a relay is opened so that the windings of the electric motor are disconnected from one another. In this case, the electric motor no longer provides a torque for assisting the driver in steering maneuvers.
In order to counteract an abrupt loss of steering assistance, it is known to continue to operate the electric motor in the case that two windings of the electric motor still function faultlessly. If required, the total torque to be applied by the electric motor is reduced here. However, due to the defect of at least one of the windings, the total torque applied by the electric motor is not constant, but is lower for particular positions of the electric motor or of the steering wheel. In this case, the driver needs to apply a greater steering force until the electric motor position with the weak torque has been overcome. At that moment, a larger torque is suddenly available again, as a result of which a deflection of the steering may occur. This deflection is still further amplified in that a higher set-point torque for steering assistance is requested because of the greater steering force applied by the driver.